I know the Raptor part is because you control a plane called a "Raptor", but what is the reason for having the phrase "Call of the Shadows" in the title?

That you're a mercenary perhaps? While others dwell in the light (i.e. a peaceful life), you are called by the shadows.

Probably just because it sounds cool. lol. That's really all the reason you need when titling a game of this nature.

From the manual:

"Your engine's turbines roar as you scream out of the sky. Startled troops look up just in time to see your wing cannons blaze. You feel the plane buck slightly as your missiles streak toward their targets. A flash in the distance tells you that they have done their job - and so have you.

The Mega-Corps pay well, especially for those talented enough to survive. They call you into the shadows with dreams of wealth and..."

Vorax